Revert r312224: "[ItaniumCXXABI] Always use linkonce_odr linkage for RTTI data on MinGW"

Breaks on buildbot:
http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/4548/steps/test-check-all/logs/stdio

The test in CodeGenCXX/virt-dtor-key.cpp tests using %itanium_abi_triple;
on non-windows platforms, this resolves to the current platform triple
(where there was no behaviour change), while on windows, it resolves to
a mingw triple (where the behaviour was intentionally changed).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312229 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/ItaniumCXXABI.cpp

@@ -2998,13 +2998,15 @@ static llvm::GlobalVariable::LinkageTypes getTypeInfoLinkage(CodeGenModule &CGM,
         if (RD->hasAttr<DLLImportAttr>() &&
             ShouldUseExternalRTTIDescriptor(CGM, Ty))
           return llvm::GlobalValue::ExternalLinkage;
-      // MinGW always uses LinkOnceODRLinkage for type info.
-      if (RD->isDynamicClass() &&
-          !CGM.getContext()
-               .getTargetInfo()
-               .getTriple()
-               .isWindowsGNUEnvironment())
-        return CGM.getVTableLinkage(RD);
+      if (RD->isDynamicClass()) {
+        llvm::GlobalValue::LinkageTypes LT = CGM.getVTableLinkage(RD);
+        // MinGW won't export the RTTI information when there is a key function.
+        // Make sure we emit our own copy instead of attempting to dllimport it.
+        if (RD->hasAttr<DLLImportAttr>() &&
+            llvm::GlobalValue::isAvailableExternallyLinkage(LT))
+          LT = llvm::GlobalValue::LinkOnceODRLinkage;
+        return LT;
+      }
     }
 
     return llvm::GlobalValue::LinkOnceODRLinkage;

test/CodeGenCXX/rtti-mingw64.cpp

@@ -2,12 +2,7 @@
 struct A { int a; };
 struct B : virtual A { int b; };
 B b;
-class C {
-  virtual ~C();
-};
-C::~C() {}
 
-// CHECK: @_ZTI1C = linkonce_odr
 // CHECK: @_ZTI1B = linkonce_odr constant { i8*, i8*, i32, i32, i8*, i64 }
 // CHECK-SAME:  i8* bitcast (i8** getelementptr inbounds (i8*, i8** @_ZTVN10__cxxabiv121__vmi_class_type_infoE, i64 2) to i8*),
 // CHECK-SAME:  i8* getelementptr inbounds ([3 x i8], [3 x i8]* @_ZTS1B, i32 0, i32 0),